Dichroic materials are useful for preferentially transmitting light polarized in one direction. When a layer of dichroic material is placed in the path of a beam of randomly polarized light and the component of transmitted light polarized in one plane is of low or negligible intensity compared to the intensity of light polarized in the orthogonal plane, the transmitted light is said to be linearly polarized, and the layer of dichroic material is referred to as a linear dichroic polarizer. Dichroic polarizers may transmit light of a particular polarization over a relatively wide spectrum, such as the visible spectrum, or they may transmit light of particular polarizations in more limited ranges of wavelengths.
One class of material suitable for producing dichroic effects is the class known as pleochroic dyes. A pleochroic dye molecule is a molecule having a light absorption spectrum that varies as a function of the orientation of the molecule with respect to the polarization of incident light. Dichroic polarizers can be made by forming a layer of oriented pleochroic dye molecules on a substrate. Pleochroic dye molecules which are self-orienting when applied to a suitable substrate are known, as are pleochroic dye molecules which require the application of some other orienting means, such as blending with or otherwise combining with a second material in order to produce suitable orientation.
Pleochroic molecules can be oriented by applying them to a surface which has been unidirectionally rubbed by a mildly abrasive material, thereby producing a dichroic polarizing layer, as disclosed by Dreyer in U.S. Pat. No. 2,400,877. Dreyer further discloses that a flexible polymeric film that has been suitably stretched can also produce surface orientation capable of orienting pleochroic molecules. Japanese published patent application 53-126951 discloses rubbing a glass surface with a paper or cloth impregnated with a pleochroic dye, thereby combining the surface preparation step with the coating step. An alternative method of surface preparation is disclosed in Japanese published patent application 3-294802, wherein grooves or trenches are formed by exposing a layer of photoresist to a holographically generated line pattern, developing the photoresist, rinsing, and applying iodine or a dichroic dye to the resulting structured layer. Surface orientation processes suffer the disadvantage of requiring additional, sometimes difficult processing steps, which can be especially troublesome in a manufacturing setting. Moreover, not all materials that might be useful as substrates for dichroic layers are necessarily amenable to the particular type of surface orientation that a particular dichroic layer might require.
Substrates capable of orienting certain dyes can also be produced by stretching a polymer, such as polyvinyl alcohol, followed by dying of the polymer, as disclosed in published Japanese patent application 62-65727. As disclosed in U.S. Pat. No. 5,639,809, materials having polarizing properties can also be produced by dying a film of polyvinyl alcohol before stretching, rather than after, and then stretching the dyed film to orient the dye.
Dichroic polarizers formed by combining a dichroic dye with a monomer, coating the mixture onto a substrate, and curing by means of radiation are disclosed in U.S. Pat. No. 5,024,850. As disclosed in published Japanese application 56-064301, the dichroic dye itself can also be a polymer. As disclosed in published Japanese application 56-051701, some commonly known pleochroic dyes can be combined with suitable polymers, such as those containing tertiary or quaternary nitrogen atoms, to form, after stretching, a polarizing film. The need for stretching the substrate, either before or after application of the dye-containing layer, limits the application of such polarizing layers to those substrates which can be stretched, and which undergo the desired surface orientation as a result of stretching. Glass, for example, is a very useful substrate for many optical applications, due to its dimensional stability and durability, but glass is not readily orientable by stretching. Moreover, stretching of the substrate in a suitably controlled manner requires considerable investment in equipment and expenditure of operating resources.
Dyes are known which form ordered structures having dichroic properties without combining with other materials. Dyes which are capable of forming stable liquid crystalline phases induced by shear effects during the coating process are disclosed in published PCT application WO 94/28073, wherein sufficient ordering is produced during coating and retained during drying to produce a layer having polarizing properties. Dichroic layers formed in this manner suffer from the disadvantage that only a limited class of dye compounds are capable of ordering themselves in this manner, whereas there are a great many other pleochroic dyes which have useful color and other properties which would be useful if ways could be found to hold them in suitable orientations.
Dye molecules which absorb light at suitable wavelengths, but do not, by themselves, form ordered or polarizing layers, can, in some instances, cooperate with liquid crystalline compounds to form dichroic materials. This effect has been used to produce liquid crystal displays, as discussed, for example, by Heilmeier et al, "Guest-Host Interactions in Nematic Liquid Crystals", MOLECULAR CRYSTALS AND LIQUID CRYSTALS, (1969), Vol. 8, pp. 293-304. Guest-host phenomena in aqueous solutions of disodium cromoglycate, which forms nematic ordered structures in aqueous solution, were studied by Bostwick et al, "Polarized Absorption Spectroscopy as a Tool in Studying Guest-Host Interactions in a Nematic Lyotropic Liquid Crystal", MOLECULAR CRYSTALS AND LIQUID CRYSTALS, Vol. 147, June, 1987, pp. 179-186. The disclosed guest-host materials suffer, however, from being liquids, which is a disadvantage in applications where a solid polarizing layer is desired.